Alteration of plagioclase to nepheline in the Khariar alkaline complex, SE India: Constraints on metasomatic replacement reaction mechanisms

At the Khariar alkaline complex in southeastern India, some tholeiitic amphibolites inter-layered with syenite and nepheline syenite show microtextural evidence of fenitization. Typical metasomatic textures include the replacement of plagioclase by vein nepheline and albitic-plagioclase or by symplectitic nepheline + albite + Na–Al-rich clinopyroxene. Whole rock mass balance calculations indicate that the fenitized variants gained alkalis (Na + K), some large ion lithophile elements (Sr, Rb and Ba) and high field strength elements like Zr and Hf during the metasomatic exchange. The plagioclase replacement textures can be explained by a fluid-mediated coupled dissolution–reprecipitation process whereby the infiltration of alkali-bearing fluid along grain boundaries and deformation-induced intragranular fractures led to the dissolution of the host plagioclase along an inward moving reaction interface. Na+ ions were introduced and Ca2 + ions removed by the fluid phase. The Al and Si released from the breakdown of the anorthite component of the plagioclase combined with Na+ from the fluid to precipitate nepheline + albitic-plagioclase or Na–Al-rich clinopyroxene. Reaction enhanced cracking during metasomatism due to local volume changes in the alteration zone produced a hierarchical system of fractures. Mass-balance calculations using the volume proportion of phases in the plagioclase replacement zones show that SiO2 and Al2O3 were broadly conserved during the replacement; there was significant loss of CaO (ca. 73%) and huge gains of Na2O and K2O. These results indicate that alkali and alkaline elements (Na, K and Ca) were exchanged freely between the fluid phase and the reaction zone whereas Si and Al released during plagioclase dissolution were consumed in the replacement zone itself to form nepheline and albite-rich plagioclase or Na–Al-rich clinopyroxene. The trace element and Sr–Nd isotope signature of the rocks indicate that the fenitizing fluids were derived from the nepheline syenites.

► Fenitization of tholeiitic amphibolites at Khariar alkaline complex, SE India
► Plagioclase replaced by vein Ne + albite or symplectitic Ne + albite + Na–Al Cpx
► Textures formed by fluid-mediated coupled dissolution–reprecipitation process.
► Whole rock mass balance shows alkalis, LIL and Zr and Hf gained by fenitized rock.
► Plag grain-scale mass balance shows Na, K, Ca freely exchanged but Si, Al conserved.